The structure of newly synthesized intracellular thyroglobulin molecules.

Thyroglobulin molecules from normal rats and rats treated with propylthiouracil (PTU) were studied in the electron microscope by the negative staining technique. Intracellular molecules were prepared from exocytotic vesicles, and extracellular molecules from the high-speed centrifugation supernatant. In accord with our previous findings, extracellular thyroglobulin molecules from normal glands had an ovoid conformation, whereas extracellular molecules from PTU-treated glands had a different, cylindrical shape. Thyroglobulin molecules from exocytotic vesicles were of the ovoid variety in normal as well as in PTU-exposed thyroids. Two interpretations are discussed. One would be that PTU exerts a non-physiological effect on ovoid molecules in connection with exocytosis of thyroglobulin. The other interpretation is that thyroglobulin molecules normally undergo 2 transformations (ovoid-cylinder-ovoid) during the process of exocytosis; PTU would inhibit the second transformation and cause accumulation of cylinders in the colloid.